1. Field of the Invention
The present invention relates to heat-conductive composite material which is particularly adapted for use in the manufacture of a substrate (heat sink) for mounting a semiconductor chip thereon or a lead frame in semiconductor packages.
2. Description of the Prior Art
Recently, integrated circuit chips (hereinafter referred to as "chips") of semiconductor packages, especially those in LSI or ULSI for large-scaled (super) computers, have been directed toward elevation of the integrated degree and acceleration of the operation speed, and therefore the quantity of heat generated by the operation of the devices has become extremely large due to an increase electric power consumed.
That is, the capacity of the chips has become large-scaled and the quantity of heat generated during the operation has also become large. Accordingly, if the thermal expansion coefficient of the substrate material is significantly different from that of the material of the chip of silicon or gallium arsenide, the chip will peel off from the substrate or be broken.
For this reason, heat-diffusibility has to be taken into consideration in planning the semiconductor package, and therefore the substrate to which the chip is mounted must also have a proper heat-diffusibility. Accordingly, the substrate material is required to have a large thermal conductivity.
Specifically, the substrate material is required to have a large thermal expansion coefficient which is near to that of the chip to be mounted thereon and also to have a large thermal conductivity. Hitherto, semiconductor packages of various constructions have been proposed. For example, the constructions shown in FIGS. 9-a and 9-b are known.
FIG. 9-a is referred to, where Mo material (2) having a thermal expansion coefficient which is near to that of chip (1) is brazed and laminated with Kovar alloy material (4) having a thermal expansion coefficient which is near to that of alumina material (3) constituting the package substrate, a chip is mounted on the Mo material (2), the chip is connected to the package substrate via the Kovar alloy material (4) and heat-release fin (5) is attached to the material (4).
In the noted construction, although the danger of peeling or cracking is little because the alumina material (3) and the Kovar alloy material (4) each have a similar thermal expansion coefficient; however, an insufficient heat-releasability is achieved even with the heat-release fin (5) since the material to control the heat-releasibility is the Kovar alloy material (4), which has a low thermal conductivity.
For this reason, clad boards as well as composite materials for heat sinks, such as Cu--Mo or Cu--W alloy, have been proposed as materials capable of satisfying the antinomic requirements of having a conformity with the thermal expansion coefficient of chips while having a large thermal conductivity.
As a clad board for a heat sink, a laminate material comprising a copper sheet and Invar alloy (Co--Ni--Fe or Ni--Fe) sheet is used.
Precisely, copper has a large thermal expansion coefficient though having a good thermal conductivity, and in order to depress such high thermal expansion coefficient, Invar alloy is laminated to the copper under pressure in the construction of the clad board, whereby the longitudinal thermal expansion of the board is made conformable to the chip to be mounted thereon. Where Invar alloy sheets are laminated on both surfaces of the copper sheet under pressure to give a sandwich structure, warping by temperature elevation may be prevented.
Although the clad board of this type could have the same thermal expansion coefficient as the chip to be mounted thereon, the thermal conductivity of the board in the thickness direction is not always sufficient since it has Invar alloy like the constitution of FIG. 9-a.
A chip support has been proposed, where punching metals of Ni--Fe having a thermal expansion coefficient which is near to that of the chip to be mounted on the support have been embedded into the chip-carrying surface of the support such as Cu (Japanese Patent Publication No. 58-46073).
However, since the support has the construction where the punching metals have been embedded into one surface of the support, there is a problem of warping by the bimetal effect.
A heat-release chip support has also been proposed, where a grid of Ni--Fe having a thermal expansion coefficient which is near to that of the chip to be mounted on the support has been embedded and laminated to the chip-carrying support such as Cu (U.S. Pat. No. 3,399,332).
However, there is a danger of introducing gases or dust into the support in the manufacture thereof so that the support would swell when heated. In addition, since the support has the thermal expansion-adjusting Ni--Fe grid in the center part of the thickness of the support such as Cu, it is necessary that the thickness of the support of Cu be thin in order that the thermal expansion coefficient of the surface of the support is similar to that of the grid. In such a construction, however, the thermal transmission in the direction parallel to the surface of the support would be fairly bad, though it might be good in the direction of the thickness thereof.
A heat-conductive metal plate has also been proposed, where Cu or Al is placed between a pair of Co--Ni--Fe or Ni--Fe sheets each having a thermal expansion coefficient which is similar to that of the heat source having plural through-holes and is filled into the through-holes (Japanese Patent Publication No. 63-3741).
However, where the heat-conductive metal plate of the kind is worked or machined, there would be a danger of peeling. Additionally, where the surface of the plate is coated with Ni-plate coat so that it may be brazable, the Ni-plate coat would react with the copper to cause unevenness of the Ni-plate coat or gases or dust would be introduced into the interface between the Ni-plate coat and the surface of the metal plate to cause swelling of the metal plate when heated.
In the construction of the heat-conductive metal plate of the noted kind, the heat from the heat-generating source would differ from each other between the case where Cu is the subbing layer and the case where Co--Ni--Fe or Ni--Fe sheet is the subbing layer from the local viewpoint. Precisely, there is a problem that the heat on the Co--Ni--Fe or Ni--Fe sheet would often remain thereon and therefore the metal plate could not receive the heat uniformly.
On the other hand, Cu--Mo or Cu--W alloy substrate is made of a composite of Cu and Mo or W, which is prepared by sintering an Mo or W powder having a nearly same thermal expansion coefficient as the chip to be mounted on the support to give a sintered sheet having a large porosity and then applying a fused copper thereto (Japanese Patent Application Laid-Open No. 59-141247) or is prepared by sintering an Mo or W powder and a copper powder (Japanese Patent Application Laid-Open No. 62-294147).
The construction of the composite substrate of the kind, which is applied to a semiconductor package, is shown in FIG. 9-b, where the composite substrate (6) has flange (7) to be connected with alumina material (3) to constitute the package on the opposite side to the surface on which chip (1) is to be mounted and heat release is effected from the flange part.
Although this composite material has practically satisfactory characteristics with respect to thermal expansion coefficient and thermal conductivity, it must be machined for slicing so as to obtain the determined dimension as Mo and W each have a high density and are heavy. Such machinery working is expensive and the yield is poor.
In addition to the above-mentioned heat sink substrate, a lead frame is also required to have an improved conformity of the thermal expansion coefficient with the opposite part to be welded thereto and have an improved thermal conductivity.
In the construction of the resin-sealed semiconductor package as shown in FIG. 10, the lead frame is not only to be an electrically connecting line to the outer part but also to have an important role as a line for releasing the heat generated from the chip.
Precisely, in the semiconductor package, chip (8.sub.4) is mounted on island (8.sub.1) formed in the center part of lead frame (8.sub.0) and is fixed thereon with a brazing material, adhesive or solder, while it is electrically connected with stitch (8.sub.2) (inner lead) via bonding wire (8.sub.5) and is sealed with resin (8.sub.6) there around.
The heat to be generated from the chip (8.sub.4) reaches lead (8.sub.3) of the lead frame (8.sub.0) via island (8.sub.1), resin (8.sub.6) and stitch (8.sub.2) in order and is then released outwards.
Accordingly, the lead frame (8.sub.0) is desired to be made of a material having a high thermal conductivity as it may well release the heat generated from the chip to the outward.
On the other hand, peeling of chip (8.sub.4) from island (8.sub.1) occurring in the adhering interface therebetween and cracking in resin (8.sub.6) would be caused by the difference of the thermal expansion coefficient between the chip (8.sub.4), the sealant resin (8.sub.6) and the lead frame (8.sub.0). In order to prevent such peeling or cracking, the conformity of the thermal expansion coefficient is indispensable between the said chip (8.sub.4), resin (8.sub.6) and lead frame (8.sub.0).
As mentioned above, as the lead frame for the plastic semiconductor package, those which are made of a copper alloy having a good thermal conductivity have heretofore been employed in many cases in view of the good heat-releasability of the alloy.
However, in the field of requiring high reliability, copper alloys are not preferred, since they have a low mechanical strength and have a poor conformity with chips with respect to the thermal expansion coefficients therebetween and therefore there would be a danger of peeling of chip from island in the welded interface therebetween. For this reason, a semiconductor package made of an Ni--Fe alloy having a low thermal expansion coefficient, such as 42% Ni--Fe alloy, has been proposed in view of the conformity of the thermal expansion coefficient of the alloys with chips.
However, as Ni--Fe type allows have a poor thermal conductivity, the lead frame made of such alloys could not have a heat-releasability enough to satisfy the current requirement. In addition, the difference of thermal expansion between the chip and sealant resin is extremely large. Accordingly, even if the conformity of the thermal expansion coefficient is satisfactory between the lead frame and the chip, the conformity between the lead frame and the resin is poor so that it is difficult to completely prevent cracking of the sealant resin.
In ceramic semiconductor packages the lead frame is made of an Ni--Fe type alloy and is equipped with Al in the sealing position thereof in many cases, since the packages are sealed with glass. However, the Ni--Fe type alloys have a poor heat-releasability, as mentioned above, there is a problem on the conformity with ceramics with respect to the thermal expansion coefficient therebetween.